1. Field of the Invention
The present invention relates to a reference wheel velocity calculation apparatus which calculates the wheel velocity (hereinafter referred to as "reference wheel velocity") used as the control reference in an apparatus for electronically controlling vehicle movement with an optimized slip characteristic.
2. Description of the Prior Art
When an automotive vehicle is running on a dry asphalt surface in a stable condition, the wheels of the vehicle rotate at a speed slightly faster than the vehicle speed, with an optimized amount of spinning. In this case, the wheel velocity itself may be defined as the reference wheel velocity. The reference wheel velocity may also be defined as the vehicle speed, the wheel velocity at which there is absolutely no slipping or spinning.
When driving on an icy surface or other poor road, the measured wheel velocity tends to be lower than the reference wheel velocity when the brakes are applied because there is a tendency for the brakes to lock the wheels, or be higher than the reference wheel velocity during starting or acceleration of the vehicle because the wheels spin. To control the wheels, it is first necessary to know the reference wheel velocity. The obtained reference wheel velocity may then be used, for example, in an antilock control system.
For example, in a vehicle equipped with an antilock control system, wheel slipping and the slip rate are obtained from the reference wheel velocity and the measured wheel velocity, and when slipping becomes excessive, the braking force is relieved and the wheel speed is again increased in order to maintain optimum wheel slipping. Though accurate values may be obtained if the vehicle speed is measured with, for example, a spatial filter and the reference wheel velocity is calculated from thus measured vehicle speed, the required apparatus would be bulky and expensive, and is therefore not practical. In general, the estimated vehicle speed or reference wheel velocity is calculated (actually estimated) from a measured wheel speed.
In a method used in vehicles equipped with an antilock control system, the velocity of each wheel is measured at each control cycle dT, and the maximum among each wheel velocity can be defined as the reference wheel velocity.
However, according to this method, the reference wheel velocity is estimated to be unduly low when all four wheels show a tendency to lock.
Apparatuses which have improved upon this problem include that described in U.S. Pat. No. 3,260,555. According to this device, when the deceleration of the highest wheel velocity is greater than a specified deceleration, the wheels are assumed to be in the process of lock, and the reference wheel velocity is calculated by substituting the the maximum assumed deceleration possible for the vehicle for deceleration of the highest wheel velocity.
In another proposed prior art method (U.S. Pats. No. 3,811,739 and 4,053,188), the reference wheel velocity is set as a value normally unequal to the maximum wheel velocity, and the increase (acceleration) or decrease (deceleration) in the reference wheel velocity is determined from the amount of the difference between the reference and maximum wheel velocity.
In yet another proposed prior art method (U.S. Pat. No. 3,401,984), vehicle deceleration is measured and integrated to define the reference wheel velocity, and in another (U.S. Pat. No. 3,744,849) this integrated value is repeatedly compared and updated with the maximum wheel velocity to remove accumulated error.
While some of the previously disclosed devices according to the prior art, such as U.S. Pat. No. 4,053,188, show applicable concepts, none of the disclosed apparatuses alone are yet able to provide a reference wheel velocity sufficiently smooth and reliable for antilock-controlled wheels on various types of road surfaces including dry asphalt, ice, and other poor roads.
Particularly in four-wheel drive vehicles, which have recently become widely used, the difference between the maximum wheel velocity and the desired reference wheel velocity can more easilY become excessive due to the small difference in the wheel velocities of the four wheels.